1. Field of the Invention
The present invention relates to methods of stimulating and producing multiple stratified low permeability hydrocarbon reservoirs having numerous separate reservoir compartments.
2. Description of the Prior Art
Field areas containing multiple stratified or laminated hydrocarbon bearing formations exist in some parts of the world. Such field areas are comprised of a large number of sandstone or other permeable rock layers containing hydrocarbons that are encased or separated by shale or other relatively impermeable rock layers of varying thickness. In addition, the sandstone layers often do not extend in a homogeneous fashion over an extensive area due to lateral stratigraphic variations and structural trapping features such as sealing faults. This lateral stratigraphic variation and structural trapping coupled with the presence of impermeable rock layers create numerous separate reservoir compartments of varying size over a relatively large vertical laminated sequence and field area. In many field areas, these reservoir compartments contain large quantities of hydrocarbons.
The production of hydrocarbons from multiple stratified hydrocarbon reservoirs has heretofore been a low economic return venture for oil and gas exploitation companies even when significant oil and gas has been confirmed to be in place. The problem is that the hydrocarbons are contained in numerous relatively small reservoir compartments, many of which cannot be practically or economically penetrated by well bores. The problem is further complicated by the fact that the reservoir formations containing the hydrocarbons have relatively low permeabilities.
Heretofore, attempts have been made to produce the low permeability reservoir compartments of multiple stratified reservoirs by way of hydraulic fracture stimulated wells. These well stimulation treatments involve the injection of viscous fracturing fluids into subterranean formations at rates and pressures sufficient to fracture the formations. Proppant material, such as sized sand, is mixed with the fracturing fluid to keep the created fractures open after the fracturing process is concluded. In most cases, the fractures formed in the stratified hydrocarbon bearing formations are vertically oriented and extend outwardly from the well bore in a direction perpendicular to the least principal formation stress in the horizontal plane.
Due to variances and uncertainties related to rock mechanical properties and formation pore pressures in the sandstone reservoir compartments and the shale intervals that encase the sandstone, attempts to propagate fractures through the compartments has yielded unpredictable and often poor results using prior art practices. Furthermore, problems have historically been experienced in propping shale intervals located between more permeable sandstone formations due in part to the lack of fracture fluid leak off adjacent to the shale intervals. Soon after fracturing fluid injection operations are concluded and during fracture closure, the fracturing fluid tends to migrate toward the sandstone formation intervals as the fluid portion of the fracturing fluid leaks off causing relatively low proppant concentration adjacent to the shale intervals. This fracture width pinching phenomena is often compounded by increased proppant embedment adjacent to the shale intervals. The resulting poor conductivity of the propped fracture adjacent to the shale intervals impedes the desired commingling of the separate reservoir compartments into one well.
In crude oil bearing multiple stratified formations, the highly compartmentalized reservoirs are typically solution gas driven whereby the predominant reservoir energy causing the crude oil to move toward production wells completed in the reservoirs is the expansion of the gas in solution with the crude oil under pressure. Typically, after a relatively small percentage of the oil in the reservoir has been produced, the reservoir pressure declines to a level allowing the gas to break out of solution from the crude oil and become free natural gas in the reservoir. Because the viscosity of natural gas is much less than the viscosity of liquid crude oil, the natural gas bypasses the crude oil as it preferentially flows through the reservoir toward the production wells. This is detrimental to the efficient production of the more valuable crude oil because of the loss of the gas drive. Gas breaking out of solution with the crude oil in the reservoir also adversely effects the relative formation permeability to the crude oil as is well known by those skilled in the art of reservoir engineering.
The recovery efficiency of solution gas drive oil reservoirs is relatively low unless secondary or enhanced oil recovery processes are employed, i.e., unless certain gases, steam, chemicals and/or water are injected from specially equipped wells completed at strategic locations in the reservoir to flood, sweep or otherwise drive the crude oil toward the production wells and/or to maintain reservoir pressure at a high enough level whereby the gas remains in solution with the crude oil. Unfortunately, due to the relatively small size of each reservoir compartment and the heterogeneous nature of the hydrocarbon containing formations in most multiple stratified reservoirs, secondary recovery and enhanced oil recovery processes have not been effective using prior art methods.
Thus, there is a need for improved methods of stimulating and producing multiple stratified hydrocarbon reservoirs whereby effective propped fractures are created in the formations which commingle various reservoir compartments and allow the reservoirs to be efficiently produced. Further, in cases where such stratified reservoirs primarily contain crude oil, there is a need for such methods whereby the crude oil is produced by gravity drainage and solution gas expansion drive in combination with enhanced oil recovery processes enabling a larger percentage of the oil originally in place to be recovered at a lower cost per barrel of oil produced.